1. Field of the Invention
This invention relates to a microwave antenna assembly suitable as an antenna for marine radars, and more particularly to a slotted waveguide antenna assembly provided with a dielectric beam-shaper so as to be compact and light in weight. The term "dielectric beamshaper" is used here to mean a dielectric arrangement comprising a pair of dielectric plate members for guiding electromagnetic waves not therethrough but therebetween.
2. Prior Art
A slot array antenna assembly comprising a slotted waveguide array is already known This type of antenna assembly is more compact and lighter in weight than a parabolic reflector antenna and therefore it is widely used for marine radars or the like. However, the slotted waveguide can provide a beam directivity only in a plane including an axis of the slotted waveguide (hereinafter referred to as a horizontal plane) and cannot provide a beam directivity in a plane perpendicular to the horizontal plane (hereinafter referred to as a vertical plane). For this reason, the antenna assembly of this type is usually provided with a flared horn to obtain a directive beam in the vertical plane. Such arrangements have been disclosed in U.S. Pat. No 2,730,717.
In order to obtain high directivity in the vertical plane, the conventional slot array antenna assembly needs a flared horn of large diameter in that plane, though it is much smaller in diameter as compared with the parabolic reflector antenna. Therefore, the entire antenna assembly is inevitably made bulky and heavy. The large aperture diameter in the vertical plane has another drawback, namely the increase in wind pressure when the antenna assembly is used in the open air.
The wind pressure has a great influence not only upon the strength of the antenna itself but also upon the rotational driving system of the same in open seas where winds are stronger. In this connection, it should be noted that the detecting ability of the radar is increased as the antenna is raised higher. However, if the heavy antenna is located at a high position on a ship, the righting ability of the ship is decreased and the ship may be capsized in rough seas. For this reason, it is very important for marine radar antennas to be light and to reduce the wind pressure.
Various attempts have been made to increase the directivity within the vertical plane without using a flared horn having a large-diameter aperture. For example, U.S. Pat. No. 3,234,558 to Borgiotti discloses a slot antenna assembly using dielectric members.
The antenna assembly of U.S. Pat. No. 3,234,558 includes, as a wave source, a so-called shunt-edge slotted waveguide in which slots are formed through the narrow side hereof. Two metal, conductive plates are disposed in parallel so that the guide may be held centrally in a space defined therebetween. A radiating dielectric structure is fitted to the forward ends of the plates, namely, fitted in an opening formed by the two plates. This radiating dielectric structure is formed in a U-shape with the base end portions connected to the plates and the curved portion projecting forwardly.
Although the operating principle and characteristics of the antenna assembly can not be known from the specification of the U.S. Patent because the specification does not refer to the propagation mode and the radiation pattern of the antenna assembly, it may be inferred from the structure as illustrated in the drawing that the antenna assembly according to the U.S. Patent utilizes the radiation in a mode of propagation between the dielectric plate member.
More specifically, the antenna assembly has a radiating dielectric structure with a circular half cylindrical closure, which is considered to be for preventing reflection of a progressive wave to enhance the radiation efficiency. This shows there is an electromagnetic wave propagation mode between the dielectric plate member of the U-shape and between the dielectric plate member. A considerable portion of electromagnetic wave energy is transmitted to the forward end portion of the radiating dielectric structure through the dielectric plate member and radiated from the end portion into the air.
In the antenna assembly of the U.S. Patent, however, there is discontinuity in the impedance at joint portions of the two metallic conductive plates and the radiating dielectric structure an electromagnetic wave is also radiated from the joint portions.
Thus, the antenna assembly of the U.S. Patent is considered to have two wave sources at the forward end and base end portion of the radiating dielectric structure.
The antenna assembly having two wave sources provides a synthesized radiation pattern formed of radiations from the two wave sources. However, the electromagnetic wave generated due to such mismatching impedances is not radiated in a direction of an extension of the radiating dielectric structure and forms a large side lobe. Thus, the conventional antenna assembly can never assure the reduction of side lobes theoretically and can not realize side lobe suppression experimentally, either.
This can also be seen from the fact that no antenna assembly as disclosed in U.S. Pat. No. 3,234,558 have been successfully put into practical use or put into the market. The operating principle of this type of the antenna assembly has not sufficiently been known.
With a view to overcoming these problems, some of the inventors of the present invention previously proposed a slotted waveguide antenna assembly provided with a dielectric wave-guiding arrangement and a smallsized reflector which is used in place of the flared horn to reduce the diameter of the aperture thereof in the vertical plane for reducing the size and weight of the antenna assembly and for reducing the counter-wind area of the antenna assembly (U.S. Ser. No. 350,739 now U.S. Pat. 4,488,157).
FIG. 21 illustrates one form of the slotted waveguide antenna assembly proposed by them. In the slotted waveguide antenna assembly of FIG. 21, a web of a holder member 12 is formed in a channel-shape and has a groove 12a. A slotted waveguide 10 is fitted and fixed in the groove 12a. A dielectric wave-guiding arrangement 16 comprising a pair of dielectric plate members 16a, 16b which are sufficiently thin as compared with a working wavelength, is provided in front of the slotted waveguide 10 so as to extend forwardly, defining a forward space therebetween. Reflectors 20, disposed at an aperture portion of the holder member 12 on the external surfaces of the respective dielectric plate members 16a, 16b, reflect an electromagnetic wave radiated through the base portions of the dielectric plate members 16a, 16b.
With this slotted waveguide antenna assembly, a component of the electromagnetic wave radiated from the slotted waveguide 10 which has a small angle with reference to the horizontal plane, is reflected so as to be guided forwardly by the inside faces (faces opposite each other) of the respective dielectric plate members 16a, 16b. A component having a large angle is also reflected by the reflectors 20 forwardly so as to increase the electromagnetic wave energy directed forwardly for providing a directive beam and to reduce side lobes.
In this case, the reflectors 20 may be short because they are required only to reflect the component of the electromagnetic wave which is radiated from the base portions of the dielectric plate members at large angles. Therefore, an area of the aperture can be reduced as compared with the conventional flared horn. For example, the height of the aperture, which is four times that of the working wavelength in the conventional flared horn-type antenna assembly, can be reduced to about twice the wavelength in this conventional slotted waveguide antenna assembly. Thus, the actual area of the aperture can be reduced.
Although the slotted waveguide antenna assembly as illustrated in FIG. 21 is sufficient for practical usage, further reduction of the side lobe level is sometimes required.
The reduction of the side lobe level may be attained in various ways. Most simply, the side lobe level can be lowered by prolonging the reflectors and enlarging the aperture area. However, this method is not desirable because it increases the weight of the assembly and increases the wind pressure.
This type of antenna assembly involves another problem in that the side lobe level is increased to an extent which is not negligible if the dielectric plate members 16a, 16b are prolonged forwardly so as to change another characteristic of the radiated electromagnetic wave, for example, to sharpen the pattern of a main lobe in the vertical plane, i.e. to reduce the half-power beam width.
This invention has been achieved in order to obviate the problems involved in the conventional antenna assembly.
It is therefore an object of the present invention to provide a slotted waveguide antenna assembly which is capable of reducing side lobes in the vertical plane only by changing the configuration and mounting state of the dielectric plate members without increasing the entire weight of the assembly and the aperture area of the reflectors.
It is another object of the present invention to provide a slotted waveguide antenna assembly which is capable of supressing side lobes to an extent which is negligible in practical use even if they are raised by a change in characteristics of the antenna assembly.